The present invention relates to optical devices such as solid-state imaging devices, photo-receivers for use in optical pickup systems and hologram units, and also relates to methods for fabricating the devices.
In recent years, optical devices incorporated in video cameras, digital cameras, digital still cameras and others are provided in the form of packages in which imaging devices such as CCDs are mounted on adapters such as bases made of insulating materials and light-receiving regions are covered with transparent boards.
To reduce the size of optical devices, imaging devices are mounted as bare chips on adapters such as bases (see, for example, Japanese Unexamined Patent Publication (Kokai) No. 2000-58805.)
FIG. 7 is a cross-sectional view showing a structure of a conventional optical device. As shown in FIG. 7, the optical device is mainly made of a ceramic or a plastic resin and includes: a base 131 in the shape of a frame having an opening 132 in its center; an imaging element 135 mounted on the lower face of the base 131 and constituted by, for example, a CCD; and a transparent board 136 of glass attached to the upper face of the base 131 and facing the imaging element 135 with the opening 132 sandwiched therebetween.
A recess 133 is formed in the lower face of the base 131 along the periphery of the opening 132. An interconnect 134 made of a gold-plating layer is provided to cover a part of the base 131 extending from a region of the lower face of the base 131 near the opening 132 to the outer side of the base 131. The imaging element 135 is attached to a portion of the lower face of the base 131 at the periphery of the recess 133, and have its light-receiving region 135a exposed to the opening 132.
Pad electrodes (not shown) for signal transmission between the imaging element 135 and external equipment are provided on the upper face of the imaging element 135 near the outer periphery thereof. An inner terminal portion is formed at the end of the interconnect 134 adjacent to the opening 132. The inner terminal portion of the interconnect 134 and the pad electrodes are electrically connected to each other with bumps 138. The imaging element 135, the interconnect 134 and the bumps 138 are sealed with a sealing resin 137 covering the periphery of the imaging element 135 on the lower face of the base 131.
As described above, the light-receiving region 135a of the imaging element 135 is located in a confined space formed in the opening 132. As shown in FIG. 7, this optical device is mounted on a circuit board with the transparent board 136 facing upward. An outer terminal portion is formed in a part of the interconnect 134 extending off the recess 133 and located on the lower face of the base 131. This outer terminal portion is used to establish connection to an electrode on the circuit board.
Though not shown in FIG. 7, a barrel including an imaging optical system is placed above the transparent board 136. The positional relationship between the barrel and the light-receiving region 135a is defined to have its required accuracy within a given tolerance.
Light from an object whose image is to be captured is concentrated on the light-receiving region 135a of the imaging element 135 through the imaging optical system incorporated in the barrel and is subjected to photoelectric conversion by the imaging element 135.
Unlike the structure of the base 131 shown in FIG. 7, another optical device using a base having a flat board shape as a whole, i.e., a base in which the recess 133 is not formed in the face on which the imaging element 135 is mounted, is also known (see, for example, Japanese Unexamined Patent Publication (Kokai) No. 2002-43554.) In this device, an outer terminal portion located at the outer periphery of the base extending off the edge of an opening in the base is connected to an electrode on a circuit board via, for example, solder balls with large diameters. The distance between the lower face of an imaging element and the upper face of the circuit board is adjusted using these solid balls.
The solid-state imaging device with such a structure has a small height as a package and has a small footprint. Therefore, this device is suitable for high-density packaging.
Structures basically similar to the foregoing structures are adopted in other optical devices, e.g., photo-receivers for use in optical pickup systems for performing, for example, writing, reading and rewriting of information on recording media such as DVDs, CDs and MDs and hologram units in each of which a plurality of elements in an optical pickup are integrated.
However, the structure of the conventional optical device shown in FIG. 7 does not exhibit a sufficient integration density as a whole system such as a solid-state imaging device or an optical pickup, and thus is susceptible to improvement.